The present invention relates to an image forming apparatus such as a printer, a facsimile or a copying machine to form an image by using an electrophotographic technique, and an image forming method.
An image forming apparatus using an electrophotographic technique includes an image carrier having a photosensitive layer on an outer peripheral surface, a charging unit for uniformly charging the outer peripheral surface of the image carrier, an exposing unit for selectively exposing the outer peripheral surface of the image carrier charged uniformly by the charging unit to form an electrostatic latent image, a developing unit for giving a toner to the electrostatic latent image formed by the exposing unit to form a toner image, and a transfer unit for transferring the toner image developed by the developing unit to a transfer medium such as a paper. There has been known an image forming apparatus for primarily transferring the toner image on the image carrier to an intermediate transfer belt and secondarily transferring the toner image on the intermediate transfer belt to a transfer medium in order to transfer the toner image developed on the image carrier to the transfer medium.
Some intermediate transfer belts have a single layer structure formed of a dielectric. The intermediate transfer belt of this type is pressed in contact with an image carrier formed by two transfer rollers having a conductiveness, and furthermore, a voltage having a reverse polarity to the polarity of a toner on the image carrier is applied. In the intermediate transfer belt having the single layer structure formed of the dielectric, a great potential difference is made between the two transfer rollers and a pressure contact portion (a transfer portion) with the image carrier to be an intermediate portion. For this reason, if an electric field from which a sufficient transfer efficiency is obtained is to be formed in the pressure contact portion (transfer portion) with the image carrier, the potential difference between a contact portion with the two transfer rollers in the intermediate transfer belt and the image carrier is excessively increased so that a discharge in this portion is generated. Consequently, there is a problem in that toner scattering is generated due to the discharge and the quality of an image is thus influenced. Moreover, some distance is made between the two transfer rollers constituting an electrode portion and the pressure contact portion (transfer portion) with the image carrier, and an unevenness is easily generated on the electric field in the transfer portion by the influence of the unevenness of the surface resistance of the intermediate transfer belt itself. As a result, there is a problem in that a transfer unevenness is easily generated.
For a countermeasure, there has been developed an intermediate transfer belt having a multilayer structure which is constituted by a conductive layer and a resistive layer formed on the conductive layer and serves to press the resistive layer in contact with an image carrier. The intermediate transfer belt having the multilayer structure including the conductive layer and the resistive layer can apply a uniform electric potential over the whole region of the pressure contact portion of the image carrier with the intermediate transfer belt. Therefore, it is possible to suppress toner scattering caused by a discharge and the generation of a transfer unevenness due to the unevenness of a surface resistance which are the problems of an image forming apparatus using an intermediate transfer belt having a single layer structure which is formed of a dielectric.
Moreover, it has been known that the image forming apparatus using the electrophotographic technique has a process control unit for properly regulating image density control factors (an exposure energy, a non-image portion potential, an image portion potential and a developing bias potential) in such a manner that the image density is optimized also in various use environments (a temperature and a humidity). In an actual image forming process, however, a toner image is formed with these factors related mutually. For this reason, these factors cannot be always controlled independently and optionally. In the factors, the absolute value of a potential difference between a developing bias potential Vb and a non-image portion potential Vd on an image carrier will be referred to as a reverse contrast potential Vr. In the case in which Vr=|Vb−Vd| is set, toner scattering is increased and a fog is also increased when Vr is small. On the other hand, when Vr is great, both the amount of the toner scattering and that of the fog are decreased and a toner is stuck, with difficulty, to an image portion in a narrow region interposed between non-image portions, particularly, in an electrostatic latent image on an image carrier. As a result, there has been known a deterioration in the quality of a low density image having a comparatively low area ratio of a dot, for example, a blur is generated on an isolated dot or a fine line and the uniformity of a line width is damaged. As a countermeasure, there has been proposed an image forming method of holding the reverse contrast potential Vr to be the absolute value of the potential difference between the developing bias potential Vb to be applied to a developing unit and the non-image portion potential Vd on the image carrier to be constant, and furthermore, forming a halftone toner image while setting and changing the image density control factor to influence the image density of a toner image in a multistage, and optimizing the image density control factor based on the result of the detection of the image density of the toner image, thereby optimizing the image density of the toner image. According to the image forming method, it is possible to cause the image density to be proper while preventing toner scattering into the image forming apparatus by holding the reverse contrast potential Vr to be the absolute value of the potential difference between the developing bias potential Vb and the non-image portion potential Vd on the image carrier to be a proper value (see for example, JP-A-11-153910 and JP-A-2003-215862).
Moreover, a corona charger is generally used as a charger unit for charging an image carrier. There has generally been known a corona charger having a discharge electrode provided in a back plate to be a metal casing and having a grid electrode provided between the image carrier and the discharge electrode to apply a grid bias potential Vg to a grid electrode in order to apply a high voltage Va to the discharge electrode to generate a corona discharge and to uniformly charge the surface of the image carrier. The grid bias potential Vg and the non-image portion potential Vd in the transfer portion of the image carrier have a functional relation (see, for example, JP-B-7-21671).
Furthermore, there has been developed a corona charger for increasing a charge current stepwise based on information about a lifetime such as the number of times of use in order to prevent a deterioration in an image due to the contamination or aging caused by a toner in a discharge electrode, a grid electrode or a back plate.
In the image forming apparatus using the intermediate transfer belt having the multilayer structure including the conductive layer and the resistive layer, however, in some cases in which Vdt=|Vd−Vt1| is increased, an abnormal image which is not transferred partially or wholly is generated in a primary transfer portion, wherein a potential difference between a non-image portion potential Vd on the image carrier and a primary transfer bias potential Vt1 is represented by Vdt. In particular, this phenomenon is remarkably presented over a half image.
This phenomenon is presented for the following reason. More specifically, when the potential difference Vdt between the non-image portion potential Vd and the primary transfer bias potential Vt1 is equal to or greater than a threshold Vth, an abnormal discharge is generated locally and instantaneously between an image carrier provided before a transfer nip and the intermediate transfer belt having the multilayer structure including the conductive layer and the resistive layer so that a necessary transfer potential cannot be obtained. This can be confirmed from the fact that a toner to be moved onto the intermediate transfer belt from the surface of the image carrier is rarely transferred to a primary transfer nip portion at time of the generation of an abnormal image but remains on the image carrier.
Furthermore, the phenomenon in which the abnormal image is generated due to the abnormal discharge is not confirmed in the image forming apparatus using the intermediate transfer belt having the single layer structure formed of a dielectric at all, and therefore, is peculiar to the image forming apparatus using the intermediate transfer belt having the multilayer structure including the conductive layer and the resistive layer. For this reason, the abnormal discharge according to the invention is completely different from a discharge before the transfer nip which causes the toner scattering in the image forming apparatus using the intermediate transfer belt having the single layer structure formed of a dielectric, and an influence on the quality of an image is greater than a deterioration in the quality of an image which is caused by the toner scattering due to the discharge generated in the image forming apparatus using the intermediate transfer belt having the single layer structure formed of the dielectric beyond comparison.
Referring to the abnormal discharging phenomenon; it has been found that the threshold Vth of the potential difference Vdt at which an abnormal image is started to be generated by an abnormal discharge between the non-image portion potential Vd and the primary transfer bias potential Vt1 is changed depending on a variation in the thickness of the photosensitive layer of the image carrier, and the threshold Vth is decreased when the thickness is reduced. For example, if the threshold Vth of Vdt at which the abnormal image is started to be generated due to the abnormal discharge in 25 μm of the thickness of the photosensitive layer in the image carrier is 1000V, the threshold Vth is reduced to 950V if the thickness of the photosensitive layer in the image carrier is decreased to 20 μm.
Referring to the phenomenon of the generation of the abnormal image due to the abnormal discharge, moreover, it has been found that the threshold Vth of Vdt at which the abnormal image is started to be generated due to the abnormal discharge is changed depending on a variation in an air pressure, and the threshold Vth is decreased when the air pressure is dropped. For example, if the threshold Vth of Vdt at which the abnormal image is started to be generated due to the abnormal discharge with an air pressure of 760 mmHg (corresponding to an altitude of 0 m) is 1000V, the threshold Vth of Vdt at which the abnormal image is started to be generated due to the abnormal discharge with an air pressure of 560 mmHg (corresponding to an altitude of 2500 m) is reduced to 950V.
Referring to the phenomenon of the generation of the abnormal image due to the abnormal discharge, moreover, it has been found that the threshold Vth of Vdt at which the abnormal image is started to be generated due to the abnormal discharge is changed depending on a variation in a temperature and humidity, and the threshold Vth is decreased at a high temperature and a high humidity. For example, if the threshold Vth of Vdt at which the abnormal image is started to be generated due to the abnormal discharge at a temperature of 15° C. and a humidity of 35% is 1000V, the threshold Vth of Vdt at which the abnormal image is started to be generated due to the abnormal discharge at a temperature of 30° C. and a humidity of 85% is reduced to 950V.
Also, there is a problem in that the non-image portion potential Vd on the surface of the image carrier having the functional relation with the grid bias potential Vg is also increased and the threshold Vth of Vdt at which an abnormal image is started to be generated due to an abnormal discharge is thus exceeded, resulting in the generation of the abnormal discharge if a charge current is increased stepwise based on the number of times of use in order to prevent a deterioration in an image due to a contamination after the endurance of a discharge electrode, a grid electrode and a back plate in a corona charger.